After a scoreless opening half at HBF Park on Sunday evening, the Australians secured a deserved breakthrough five minutes after the break when Remy Seimsen set Ellie Carpenter up to float a cutback to Michelle Heyman, who placed a side-footed volley into the net.
But a late flurry from the Slovenians was rewarded a minute into added time when an effort from Naja Poje Mihelic bounced off the post and back into the path of fellow substitute Kastelec, who slid in front of a desperately retreating Australian defence to fire home.
It dampened a performance from the Australians that showed signs of improvement as they continue to adapt to the new possession-based principles of Montemurro – even if the margin was well off the 3-0 victory they secured over the same opposition on Thursday evening.
After a midweek win characterised by struggles with the Slovenian press and sloppy turnovers, the hosts were cleaner with the ball in their defensive third in the re-match, which resulted in fewer opportunities off giveaways for the Europeans.
This was accompanied by improvements in the side's movement and combination play going forward, particularly from Carpenter and Amy Sayer on the right.
The promising build-ups, however, were frequently let down by the final ball or stymied by desperate last-gasp defending.
That was until the 50th minute, when Seimsen's inch-perfect ball for Carpenter set the wingback up to put it on a plate for Heyman.
Charli Grant came close to killing the game off in the 70th minute but had her shot cleared off the line, before the visitors raised their intensity.
Having hardly been sighted to that point, Mackenzie Arnold was called into action in the 90th minute to make a quick reaction to deny Kastelec, but the Australian keeper could do little about the 22-year-old's effort a minute later.
Seeking to bounce back, Montemurro will now shuffle his deck for fixtures against Panama on July 5 and July 8, with Carpenter, Cooney-Cross and Kennedy amongst eight players leaving camp, with eight new players set to jet into West Australia.